Cup-shaped grinding wheels are well known in the manufacture of precision gearing. When such grinding tools become worn, their surfaces are dressed (renewed) to their original cutting condition by using dressers, generally diamond dressers.
Such dressers are of either the "form" or "generating" type. In form-type dressing, the outer configuration of the dressing roller is shaped exactly to conform to a working surface of the grinding wheel being dressed so that, during the dressing process, the dressing tool contacts the grinding wheel along a line of contact across the entire working profile of the surface being dressed. Such form-type dressers are quite expensive and, by necessity, must be specially made to conform to a single given wheel profile.
In contrast, dressers of the generating-type effectively contact the grinding wheel at a single "point" which, by controlling the motion of the dressing tool relative to the grinding wheel, can be moved to "generate" any desired configuration for the working surface of the wheel. Such known generating-type dressing tools are often rollers formed in the shape of small dishes or thin disks with diamond grit embedded along narrow circumferential edges. Such tools are mounted in apparatus for producing the desired relative generating motions. Generally, known generating-type dressing systems initially orient the roller with its axis aligned with the working surface on one side (e.g., inside) of the cup-shaped grinding wheel, moving the roller in a direction parallel to the working surface for generating the required profile. Then, for dressing the working surface on the other side (e.g., outside) of the cup-shaped grinding wheel, the dressing roller is pivoted to a new orientation with its axis aligned with the other working surface and is rotated in the opposite direction relative to the direction of rotation of the grinding wheel.
It is generally understood in the art that, in order to dress both the inside and outside working surfaces of the wheel with substantially similar grinding characteristics (e.g., sharpness), it is necessary to provide substantially the same relative motion between the cutting surface of the dressing roller and each of the respective working surfaces of the grinding wheel. For most known systems, this requires that the rotation of the roller be reversed for dressing the respective inside and outside working surfaces of the grinding wheel. However, when a dressing roller is rotated in a first direction during the dressing operation, its dressing surface (usually diamond grit) forms a distinctive wear pattern so that, when the roller is thereafter rotated in the opposite direction to dress the other side of the grinding wheel, the initial wear pattern tends to cause the abrasive grit to break free from the roller, thereby reducing the useful life of the roller and the overall quality of the dressing operation. Also, known generating-type dressing rollers tend to wear rapidly because the zone of contact between the dressing roller and grinding wheel is necessarily limited to the narrow circumferential cutting surfaces of the dish-shaped or disk-shaped dressing rollers.
Recently, such prior art generating-type dressing systems have been improved by advances disclosed in U.S. Pat. No. 4,862,868. Namely, a dish-shaped dressing roller is provided with a radiused circumferential outer surface, and during dressing operations only a first portion of its dressing surface is used when contacting the inner surface of the grinding wheel, while a second and different portion of the dressing surface is used to contact the outer surface of the grinding wheel. Further, this dish-shaped dressing tool is oriented so that its direction of rotation remains the same when dressing both the inner and the outer surfaces of the grinding wheel.
However, this recent improvement in the dressing of cup-shaped grinding wheels, as disclosed in U.S. Pat. No. 4,862,868, still has important problems. First, the dish-shaped roller design limits the width of the working surface which can be dressed on the inside of a cup-shaped wheel. This limitation results from interference between the outer edge of the grinding wheel and the base of the dish-shaped roller. Secondly, the dish-shaped roller is expensive and difficult to manufacture due to its relatively complex shape and the thinness of its outer circumferential dressing surface. The thinness of this dressing surface, which must be plated in a very narrow mold, makes it quite difficult to obtain an even distribution of diamonds on the roller's working surfaces. Finally, this recent prior art improvement still does not overcome a major problem which affects all prior art dressing systems and which limits the ability to achieve similar cutting characteristics on the inner and outer surfaces of the wheel. Namely, this major problem arises from the fact that, when the same dressing roller is used to dress both the inside and outside surfaces of the grinding wheel, the roller has significantly different "effective" radii of curvature relative to each of the respective sides of the wheel.
If a dressing roller has a relatively small effective radius of curvature relative to the surface of the grinding wheel, the dressing operation produces a relatively sharp grain structure on the wheel. On the other hand, a relatively large effective radius of curvature results in more polished and duller cutting grains on the surface of the grinding wheel. With prior art dressing systems, there is a significant disparity in the effective radii of the dressing roller relative to the inner and outer surfaces of the cup-shaped wheel being dressed, and this results in significant differences in the grinding characteristics of these inside and outside working surfaces.